[docs] add posthog config to doc site

[zihehuang]

Description

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Test plan

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Release notes

Check each box that your changes affect. If none of the boxes relate to your changes, release notes aren't required.

For each box you select, include information after the relevant heading that describes the impact of your changes that a user might notice and any actions they must take to implement updates.

• [ ] Protocol:
• [ ] Nodes (Validators and Full nodes):
• [ ] Indexer:
• [ ] JSON-RPC:
• [ ] GraphQL:
• [ ] CLI:
• [ ] Rust SDK:

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Name	Status	Preview	Comments	Updated (UTC)
sui-docs	🛑 Canceled (Inspect)			Jun 24, 2024 11:22pm

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[zihehuang]

@ronny-mysten Plz review 🙏